Evaluation and comparison of effects of recycling on the shutter mechanism and shear bond strength (SBS) of various self-ligating brackets: An in vitro study

Background and rationale

In dentistry, self-ligating brackets (SLBs) have been around for a long time. They have been used since the 1930s as a pioneering technology.¹ In the past 30 years there has been a revival of interest in these devices, with various new inventions being developed in this field. Over the years, several advantages have been claimed over conventional bracket systems, such as reduced friction and noise levels. The most convincing prospective advantages of SLBs are a decrease in duration of treatment and a lower level of subjective discomfort associated with the procedure.² Additionally, poorer biostability is claimed to promote periodontal health and more efficient chairside manipulation. Retrospective studies have suggested that this technique has a definite advantage of reducing the overall treatment time by four to seven months. A reduction is also seen in the number of appointments.³

As self-ligating orthodontic brackets have gained popularity in orthodontic practices over the past few years, they have become a more frequent choice among patients. As the years have gone on, self-ligating brackets have been increasingly used instead of conventional brackets. As a result of numerous studies depicting their properties and characteristics, such as frictional forces, torque expression, rotation capacity, bond strength, and reconditioning procedures, self-ligating brackets have been demonstrated to be superior in all these areas.⁴

After brackets are bonded on the teeth, it can get debonded due to various factors like inadequate access, moisture contamination, or occlusal stress during masticatory function. Various recycling methods can be used to rebond the debonded brackets, like direct flaming, sandblasting, green stone, and tungsten carbide bur. Studies have reported that recycling of brackets can lead to a degradation of quality, lower bond strength and increased risk of cross-infection.⁵

There have been several studies that have compared the shear bond strength (SBS) of orthodontic brackets with varying base surfaces and different types of bases. There have been no studies on whether recycling affects the shutter mechanism of self-ligating brackets. Due to procedures like sandblasting and direct flaming to remove the residual composite from brackets, there is an increase in temperature which may affect the material properties of brackets, including the sliding mechanism.³ The force of closure and opening of the shutter might get affected by this change in temperature during the recycling of self-ligating brackets.

Various commercially available self-ligating bracket systems will be compared based on the force required to close and open their shutters after the recycling process.⁵ They are expected to show an increase in the amount of force which may be due to the temperature change due to direct flaming and sandblasting.⁶

Discussion

A self-ligating bracket system is designed to provide light forces that are applied to a low-friction surface in order to optimize the physiologic movement of teeth and to ensure a balanced oral environment. Studies have measured the force of shutter mechanism of new SBLs of different commercially available self-ligating brackets but not in recycled brackets. This study will focus on the comparison of change in force of opening and closure of shutter mechanism of new and recycled brackets.

Research has been done on the SBS of new and recycled conventional brackets and it has been concluded that recycling reduces SBS. This study will be based on comparison of SBS of new and recycled commercially available self-ligating brackets.

Trial design

In vitro study

Study setting

This in vitro study will be conducted in the Department of Orthodontics and Dentofacial Orthopaedics, Sharad Pawar Dental College in collaboration with Central Research Laboratory (Centre of Translation Sciences), Sawangi Wardha.

Eligibility criteria

Inclusion criteria

Exclusion criteria

Interventions

Pumice and rubber prophylactic cups will be used to clean and polish the teeth for 10 seconds followed by washing with water.

Teeth will be mounted on acrylic blocks of different colors to differentiate between the four types of commercially available brackets that are being used.

Group 1: Selfy

Group 2: 3M

Group 3: Damon

Group 4: AO

All brackets will be bonded to the teeth using the Transbond XT adhesive system. These mounted teeth will be placed under the universal testing machine for testing:

The debonded bracket will be subjected to sandblasting and flaming to remove residual resin and the teeth will be prepared again by removing resin then the brackets will be rebonded on the teeth.

The force of closure of the shutter of the self-ligating bracket and SBS will be measured again using a universal testing machine.

Primary outcome

The force of closing and opening the shutter mechanism, which is the amount of force required to open the slot of different commercially available self-ligating brackets, will be measured by attaching an opening and closing apparatus to universal testing machine. This force is expected to increase after recycling.

Secondary outcome

SBS is the maximum force which the adhesive joint can tolerate before the bracket is debonded. SBS will be calculated by subjecting them under universal testing machine and it is expected to decrease after recycling.

Sample size

Sample size formula for difference between two means:

Where

Zα = level of significance at 5% i.e., 95%

Confidence interval $=$ 1.96

Zβ is the power of test $=$ 80% $=$ 0.84

δ₁ $=$ SD of SBS in group 1 $=$ 2.9011

δ₂ $=$ SD of SBS in group 2 $=$ 3.8872

$∆=\text{Difference between}\mathrm{two}\text{means}=22.443–19.057=3.386$

K=1

n $=$ 16 teeth needed in each group

The total sample size is 64

Study reference: Kopal Agarwal et al.⁷

Statistical methods

All the results will be analysed statistically using SPSS version 27 software. Data for outcomes variables will be tested for normality using the Kolmogorov-Smirnov test. The comparative analysis of the outcomes shutter mechanism force and SBS will be evaluated in Newton for the force of shutter mechanism and Newton/mm² for SBS respectively. ANOVA will be used to find the significant difference between the mean of the 4 groups. A Tukey test will be used for pairwise comparative evaluation of measurements between two groups. A p-value ≤ 0.05 will be considered as significant at 5% level of significance and 95% confidence of interval.

The aim of this study will be to assess the efficacy and efficiency of commercially available self-ligating brackets. This study will help determine the quality of commercially available brackets despite recycling in terms of force required to open and close the shutter of self-ligating brackets and its SBS, to ensure good bond strength and mechanical retention even after recycling.

Dissemination

The study results will be published as an article in a journal.

Study status

Not started yet.

Ethical considerations

Ethical approval was received from Datta Meghe Institute of Higher Education and Research, Sawangi, Wardha (IEC reference number- DMIHER (DU)/IEC/2023/573).